The present invention relates to a method of blanketing a boiler with nitrogen gas to reduce the level of dissolved oxygen in boiler water.
Internally initiated corrosion fatigue cracking of the tubes in many fossil fired boilers has become a major problem. The problem is aggravated in boilers which are frequently cycled on and off. The tube failures often result in costly forced boiler outages. In addition, internal corrosion fatigue failures are of particular concern because they can occur in the nonheat absorbing portion of the waterwall tubes. Failures can, therefore, pose a major safety concern by directing steam into high traffic areas of the plant.
Although the cause of internal corrosion fatigue cracking of boiler tubes has not yet been definitely determined, evidence strongly suggests that the level of dissolved oxygen in the boiler water plays an important role. Of particular concern is the dissolved oxygen level of the water which is used for filling the boiler and the water which is used for hydrostatic tests of the boiler.
Gas blankets have been used for years in several industries for off-line corrosion protection and as a means of preventing oxygen from redissolving in water after being removed by some other method. They have not, however, been used as a means for dissolved gas removal. Gas blankets have been used on storage tanks downstream of other deoxygenation devices, such as vacuum degasifiers and oxygen stripping columns, to preserve deoxygenated water. Nitrogen gas blankets have also been used on utility boilers but only in a method of downtime corrosion control, not deoxygenation. In fact, boiler manufacturer boiler fill and start-up recommendations make no attempt to preserve the nitrogen blanket.
Numerous water deoxygenation methods and devices are currently available. A standard device for removing dissolved oxygen from boiler water has been the vacuum degasifier which operates according to Henry's law to lower the partial pressure of oxygen by lowering the total pressure.
Another device is the oxygen stripping column which uses a stripping gas to remove oxygen from water. An oxygen stripping column is disadvantageous since it is a continuous flow process requiring the continuous supply of a stripping gas and, thus, is a relatively costly process.
Deaerators or D/C heaters have been used for years in the utility industry for the removal of dissolved oxygen from water. Steam bubblers are relatively new to the industry. Both the deaerator and the steam bubbler remove dissolved oxygen by scrubbing water with a continuous supply of steam. Oxygen removal with these devices is governed not only by Henry's Law but also by the heating of water above the desired dissolved oxygen saturation temperature. These devices suffer from the same cost disadvantage of an oxygen stripping column since it requires a continuous supply of steam.
Additionally, several methods have been devised in which an inert gas is sparged through water for dissolved gas removal. Some of these methods are more closely related to the oxygen stripping columns and, thus, are disadvantageous because they must rely on the continuous supply of a fresh stripping gas. In other gas sparging methods, the inert gas is recycled. All of the gas sparging methods rely on the circulation of an inert gas to provide the turbulence and surface contact area needed to enable the inert sparging gas to remove oxygen from water. Another disadvantage of these methods is the probability of gas binding in a boiler water circulation pump.
Although several of these methods and devices can effectively lower the dissolved oxygen level in water to less than 50 ppb, they can be quite expensive. For example, the capital investment alone for a 300 gpm to 500 gpm vacuum degasifier can range from $250,000 to $350,000 excluding installation.
Even after the investment is made in an effective make-up deoxygenation method or device, the boiler plant is still faced with the dilemma of transporting deoxygenated feedwater to the boiler without redissolving significant amounts of oxygen. It has been demonstrated that the dissolved oxygen levels of water used to fill an air-drained boiler essentially end up saturated at 7 ppm to 10 ppm (7000 ppb to 10000 ppb) regardless of the initial dissolved oxygen level. None of the above-identified methods and devices provide for the maintenance of deoxygenated feedwater supplied to the boiler.